1. Field of the Invention
The present invention relates to a reversible thermosensitive recording material capable of recording and erasing images repeatedly by utilizing its property that the transparency can be changed reversibly from a transparent state to an opaque state, and vice versa, depending upon the temperature thereof, and a method of producing the same.
2. Discussion of Background
Recently attention is paid to a reversible thermosensitive recording material capable of temporarily recording images thereon and erasing the same therefrom when such images become unnecessary. As representative examples of that kind of reversible thermosensitive recording material, there are conventionally known reversible thermosensitive recording materials in which an organic low-molecular-weight material such as a higher fatty acid is dispersed in a matrix resin such as vinyl chloride - vinyl acetate copolymer with a glass transition temperature (Tg) of 50.degree. C. to less than 90.degree. C., as disclosed in Japanese Laid-Open Patent Applications 54-119377, 55-154198, 63-39376 and 63-107584.
In the case where only heat energy is applied to the reversible thermosensitive recording material by using a heat-application roller or a heat-pen, with slight application of pressure thereto, in order to perform recording and erasing operations, the durability of the recording material is not degraded even though the image formation and the erasure are repeated. In contrast to this, when heat and pressure are applied to the recording material at the same time by using a thermal head, the durability of the recording material is degraded during the repeated operations. This is because the matrix resin around the organic low-molecular-weight material particles in the recording layer is deformed and the particle size of the finely-divided organic low-molecular-weight material particles dispersed in the matrix resin is increased while the recording and erasing operations are repeated. As a result, the effect of scattering light is decreased, and the whiteness degree of a white opaque portion in the recording layer is also decreased. In the end, the image contrast is disadvantageously lowered.
When these conventional reversible thermosensitive recording materials are manufactured, an organic solvent such as tetrahydrofuran is employed as a base solvent which dissolves or disperses both the matrix resin and the organic low-molecular-weight material for the recording layer. Organic solvents of this kind have extremely low boiling points and high evaporation rates, so that such solvents are evaporated from a coating liquid applied to a support for the formation of a recording layer thereon even before the coating liquid applied is dried. As a result, a thin matrix resin layer is formed on the surface of the recording layer. Due to the formation of the thin film of the matrix resin on the surface of the recording layer, not only the evaporation of the solvent within the recording layer is prevented but also the particle diameter of the organic low-molecular-weight material dispersed in the matrix resin increases, and the organic low-molecular-weight material is separated out on the surface of the recording layer.
Moreover, there is a disadvantage that the adhesion strength cannot be maintained at the interface between the support and the thermosensitive recording layer because of the residual solvent in the thermosensitive recording layer. In addition to the above disadvantage, because of the presence of the residual solvent in the thermosensitive recording layer, there is a problem that another layer cannot be smoothly overlaid on the surface of the recording layer. In the case where image formation and the erasure are repeated many times with simultaneous application of pressure and heat to the recording material, small particles of the low-molecular-weight material are separated out on the surface thereof. When a protective layer is overlaid on the recording material, the small particles of the organic low-molecular-weight material in the recording layer migrate to the protective layer and the small particles thereof in the form of dust contact a thermal head, and adhere thereto. The result is that the image formation and the erasure cannot be repeated many times with such a conventional reversible thermosensitive recording material.
A conventional reversible thermosensitive recording material comprises the matrix resin and the organic low-molecular-weight material at a weight ratio in the range of (1:2) to (16:1). In the case where the weight ratio of the matrix resin to the low-molecular-weight material exceeds the above range, it is difficult for the recording layer to assume a white opaque state, although the durability of the recording layer may be improved. On the other hand, in the case where the ratio of the matrix resin to the low-molecular-weight material is smaller than the above range, the durability of the recording layer is degraded and the performance of forming a film in which the organic low-molecular-weight material is dispersed in the matrix resin is degraded. A satisfactory reversible thermosensitive recording material has not been obtained yet.